The present invention relates to data communications and, more particularly, to mobile data communications.
Today, the North American cellular system is an analog system, sometimes referred to as AMPS (Advanced Mobile Phone Service). The AMPS system has been standardized in a number of Telecommunications Industry Association (TIA) standards, e.g., TR-45.1, and is based upon analog frequency modulation (FM) technology. Each cellular radio channel is a cellular carrier that is frequency modulated by one voice signal. The bandwidth of each cellular radio channel is 30 kilo Hertz (kHz).
However, the current rapid growth of cellular voice communications is straining the existing AMPS system because of the AMPS limitation of carrying only one voice signal per cellular radio channel. As a result, the telecommunications industry is looking into different approaches that that will make the limited radio frequency (RF) spectrum currently allocated to cellular systems more efficient. Although there is an interim analog-based proposal termed Narrowband AMPS (NAMPS), those in the art generally view any long-term solutions as being based upon digital transmission concepts.
One digital system is based upon time division multiple access (TDMA) techniques and is defined in TIA interim standard (IS)-54. Generally speaking, in this TDMA approach each cellular radio channel now carries three digital voice channels, where each voice channel is assigned to a separate time slot within the 30 kHz RF channel to handle different conversations. The raw data rate is 13 K bits per second for each time slot. The digital bit streams, which represent voice signals, are encoded (compressed), interleaved, and transmitted over the air using a digital modulation scheme called quadrature phase-shift-keying (DQPSK). The combination of digital modulation, error-correcting codes, and time-slot interleaving, reduces the effects of the most common radio propagation impairments. This, in turn, triples the voice channel capacity without requiring additional RF-spectrum, increases subscriber capacity, and makes the limited RF spectrum currently allocated to cellular systems more efficient.
Another digital approach is the code-division multiple access (CDMA) approach defined in TIA standard IS-95 that uses spread-spectrum technology. However, whatever digital approach is eventually adopted, the next-generation cellular system, e.g., as defined in TIA standard IS-54, will be a "dual-mode" system that supports both existing AMPS mobile phones and the new digital cellular access equipment. This dual mode approach provides current mobile phone users, and cellular service providers, with a migration path to the new digital cellular technology. As a result, new mobile phones will be designed to support both analog-based AMPS technology and the digital, e.g., TDMA, technology. In addition, dual-mode mobile phones will receive signaling from the cellular network as to whether or not to switch between the analog mode and the digital mode during the existence of a voice conversation. This allows a cellular user to move from an AMPS-based cell to a digital-based cell without interruption of an existing voice conversation.
Although the primary market of cellular communications is voice communications, a growing part of the cellular market is made up of users who desire to communicate data through the cellular system. Although there are several methods for passing data over analog cellular, the primary method used today is where a data communications equipment (DCE), e.g., a modem, couples data terminal equipment (DTE), e.g., a lap-top personal computer, to a mobile phone through an RJ11 adapter. While the modem modulates the data signal from the lap-top computer to, typically, a quadrature amplitude modulated (QAM) signal, the RJ11 adapter couples the modem's QAM signal to the mobile phone over the mobile phone's local signaling interface, which is usually proprietary. The mobile phone modulates the modem's QAM signal onto the cellular carder, which is transmitted to a cell site transceiver. The latter demodulates the cellular carder and provides a received version of the QAM signal to a Mobile Telecommunications Switching Office (MTSO) of the cellular network for transmission to the public switched telephone network (PSTN) and a far-end data endpoint. This sets up an end-to-end data communications link between the cellular data endpoint and the far-end data endpoint. The modems of each endpoint usually have a specialized protocol that provides for better operation over the adverse cellular link, e.g., ETC (TM symbol) (Enhanced Throughput Cellular).
In migrating to the digital cellular technology, the approach to passing data over cellular is different from the above-mentioned analog-based approach. In particular, the mobile phone directly couples to any DTE via a standard DTE/DCE interface like Electronic Industries Association (EIA) RS-232. This DTE/DCE interface directly provides the data in a digital form to the mobile phone. Consequently, no equivalent interface adapter, like an RJ11 adapter in the analog-based method, is necessary to pass data over a digital cellular system.
Unfortunately, although the above-mentioned "dual-mode" operation of the next generation cellular system allows a mobile phone to switch between the analog mode and the digital mode during the course of a voice call, such switching during the course of a data call drops the data connection. In particular, the dual-mode mobile phone expects data in a digital bit stream, e.g., from a DTE coupled to the mobile phone's RS-232 port. For example, when switching from the analog mode to the digital mode the mobile phone must compress the analog signal. When this analog signal represents data, e.g., a QAM-modulated signal, these voice compression algorithms distort the analog data signal to a point where the data connection is severely impaired and subsequently dropped. As a result, a cellular data connection must remain in either analog mode, or digital mode, for the duration of the data call.